A useful accelerometer must be sensitive to desired accelerations and insensitive to spurious, undesirable accelerations. For instance, an accelerometer for measuring the acceleration of a rocket must be sensitive to acceleration of the rocket in the direction of motion and insensitive to the many accelerations in other directions caused by vibration of the rocket. In addition, the accelerometer should also be capable of providing a digital output proportional to acceleration for use with conventional digital equipment.
One example of an early digital accelerometer is shown in U.S. Pat. No. 2,928,668 of Blasingame, where an accelerometer produces an output of two alternating voltages whose frequencies are related to the acceleration of the instrument. In Blasingame, a reed with a magnet on its end is caused to vibrate at its natural frequency by an electrical pickup arranged to measure the velocity of the vibrating magnet and feed its output to an amplifier which supplies current to solenoids arranged to supply side forces on the reed proportional to the magnet's velocity. Two identical reeds are arranged back to back so that the natural frequency of one is increased while the frequency of the other is decreased in response to acceleration along the longitudinal axis of the reeds. The two outputs of this system are combined to remove the unaccelerated natural frequency of this system and minimize drift due to temperature affects.
A later digital accelerometer is shown in U.S. Pat. No. 3,269,192 of Southworth, Jr., et. al. In this patent, a tuning fork has two tines clamped at each end and vibrated 180.degree. out of phase. A dense inertial proof mass is secured to one pair of ends of the tines and supported against cross-axis movement perpendicular to the tines while being free to move longitudinally so as to extend or compress the tines when the mass is accelerated.
Although Southworth does incorporate the double-ended tuning fork design of this invention, it utilizes a large mass (as opposed to the small mass of the invention) which is constrained to motion along the tine axis. No such constraints are part of the invention because the invention measures acceleration perpendicular to the tine axis. In Southworth, movement of the mass along the axis either puts both tuning forks in tension or in compression, while movement of the mass of the invention causes one tuning fork to be compressed and the other to be under tension. As a result of these differences in construction, the invention has greater sensitivity and reliability than Southworth.
Another tuning fork accelerometer is shown in U.S. Pat. No. 3,319,472, of Reefmen. In this patent, a single tuning fork having one end fixed and the other end free relies upon the physical deflection of the fork tines (under acceleration perpendicular to the tine axis) as an index of the acceleration. This deflection is sensed by magnetic pick-up coils. It is not a frequency changing effect and the accelerometer design has little resemblance to the invention.
A miniature quartz transducer which is utilized in the preferred embodiment of this invention is shown in U.S. Pat. No. 4,215,570 of Eer Nisse. In this invention a rectangular plate of quartz crystal has a continuous end portion at either end and a longitudinal slot connecting each end portion, forming a pair of parallel, spaced beams. Metal electrodes are formed on the transducer for exciting it and providing an electrical output therefrom.